SPRINGFIELD, Ill., Aug. 23 -- Sen. Barack Obama introduced Sen. Joseph R. Biden Jr. as his vice presidential running mate here on Saturday, embracing him as a "statesman with sound judgment" who had never forgotten his humble roots or lost his fundamental decency.

"Joe Biden is that rare mix. For decades, he has brought change to Washington, but Washington hasn't changed him," Obama (Ill.) said.

The announcement -- made official only hours earlier -- capped weeks of speculation and brought an infusion of experience and aggressiveness to the Democratic ticket two days before the start of the convention in Denver.

Biden, 65, a sharp-witted and energetic foreign policy expert who has held two of the most critical Senate chairmanships, bounded out onto the stage just after 2 p.m. Central time. The senator from Delaware hugged Obama and then, taking the podium as the vice presidential candidate for the first time, eagerly assumed the role of attack dog against the presumptive Republican nominee.

Biden said Sen. John McCain -- although a longtime friend -- has "yielded to the very Swift boat politics he once deplored" during the course of the current race. And he made fun of McCain for recently forgetting that he owns seven properties, saying McCain cannot understand what struggling Americans discuss over their kitchen tables.

"He'll have to worry which of the seven kitchen tables to sit at," Biden said.

Obama settled on Biden during his vacation in Hawaii the week before but did not make his choice known, even to the other contenders, until late in the week, aides said. He called Biden on Thursday night to offer him the job, catching him at the dentist during his wife's root canal procedure.

That set into motion a cloak-and-dagger operation to bring the two together here Saturday. After the appearance, Biden and his wife, Jill, conducted interviews and were photographed with Obama and his wife, Michelle. When Michelle Obama and Jill Biden met Saturday, after speaking first by phone, they had what one person in the room described as an "instant comfort level." They discussed the pending glare of the cameras at the convention and shared anecdotes about their children.

The Bidens returned to Delaware after Saturday's event and are scheduled to fly to Denver on Monday. The two families plan to campaign together again after the convention.

Biden is chairman of the Senate Foreign Relations Committee and also served as chairman of the Judiciary Committee. He commands a loyal, if small, clutch of followers in the battleground state of Iowa, having twice campaigned for president there.

His home state of Delaware has just three electoral votes, but he is originally from Pennsylvania, a critical swing state with 21. Democrats hope he can help break down any perceptions of elitism on the ticket, especially among working-class white voters.

Biden does not come without risks. Accusations that he plagiarized then-British Labor Party leader Neil Kinnock helped sink his presidential campaign in 1987. During the primaries, Biden also attacked Obama aggressively, questioning his lack of experience. Those comments had already resurfaced in a McCain ad released shortly before the announcement was made official; it also quotes Biden, in 2005, praising the Republican.

Democrats largely embraced the choice, although some supporters of Sen. Hillary Rodham Clinton expressed disappointment that Obama's top challenger in the primaries was never considered for the second slot. A senior Obama adviser confirmed that Clinton was not asked for paperwork but said it was because she had asked Obama not to vet her unless he was seriously considering her.

It appears Obama never did. A Clinton loyalist who talked to her throughout the process said the senator from New York did not want to "make him feel he had to stage a Kabuki theater process for her" by pretending to consider her if he did not want to.

"She made it crystal clear in public and in private that this was entirely his decision to make any way he wanted, choose whomever he wanted," the loyalist said. "Clearly, he had no interest. No meetings, no conversations, no requests for information, no real consideration whatsoever. And that was his prerogative."

A new McCain TV ad aims to tap into the frustration of Clinton supporters over the decision by suggesting that Obama dissed his onetime rival. "Why? For speaking the truth," a narrator says.

The ad shows clips of Clinton criticizing Obama for vaguely discussing how he would govern and for his connections to Chicago developer Antoin Rezko. "The truth hurt. And Obama didn't like it," the narrator concludes.

Clinton issued a statement praising Biden, and some close to her said the choice of Biden validated their belief that experience is an important trait in the race -- even though as a 35-year veteran of Washington he does not project an image of "change."

"Senator Obama has continued in the best traditions for the vice presidency by selecting an exceptionally strong, experienced leader and devoted public servant," Clinton said.

Obama and Biden made their joint debut in front of a crowd of tens of thousands at the old Illinois statehouse, where the presumptive Democratic nominee launched his campaign a year and a half ago. The first blue "Obama-Biden" signs were on display -- quickly produced under the cloak of secrecy as the campaign tried to keep the announcement from leaking.

In his introductory remarks, Obama described Biden's life story: as a boy raised on Catholic values in working-class Pennsylvania; as a young senator who lost his first wife and daughter in a car accident; as a senator who had fought against tyranny and injustice.

It was clear from Obama's speech that he views Biden as bringing dual assets: being able to play the role of a down-to-earth fighter, appealing to Catholics and the working class; and serving as a statesman and expert on foreign policy.

"He is still that scrappy kid from Scranton who beat the odds -- the dedicated family man and committed Catholic who knows every conductor on that Amtrak train to Wilmington. That's the kind of fighter who I want by my side in the months and years to come," Obama said.

He described Biden as being "at home in a bar in Cedar Rapids" as well as on the world stage -- two settings in which Obama has struggled over the course of the race. Scranton is of particular significance for Obama: Clinton, who also has family from the area, dominated it in the primaries.

Biden, in characteristic language, noted that some people accuse him of speaking too colloquially -- and then proceeded to do so.

"There's something about this guy, there's something about Barack Obama, that allows him to bring people together," Biden said. "It's been amazing to watch him, but then again, that's been the story of his whole life."

He said Obama has the "vision and courage" to improve the country, calling him a "clear-eyed pragmatist who will get the job done."

Biden is also running for reelection and, under Delaware law, can remain on both the presidential and Senate ballots. He is expected to cruise to reelection, and if the Obama-Biden ticket prevails, Gov. Ruth Ann Minner would appoint his replacement in the Senate.

His son, Beau, Delaware's attorney general, is considered a favorite for that potential appointment.

Throughout the selection process, the Obama campaign had kept a close watch on Virginia Gov. Timothy M. Kaine as a potential running mate. Kaine told associates that he had "very serious" conversations with Obama about joining the ticket and that he provided documents to the campaign about his background.

Speculation over Kaine reached a fever pitch last week when Obama spent two full days campaigning across Virginia and spent the night Wednesday in downtown Richmond, a few blocks from the governor's mansion.

On Friday, with a swarm of reporters parked outside, Kaine sneaked out of the mansion through an underground tunnel to help his son move to George Washington University. Kaine was notified of Obama's decision late Thursday night or early Friday, though he declined repeatedly Saturday to reveal when. He then flew to Denver.

Wayne Turnage, Kaine's chief of staff, sent an e-mail to the governor's cabinet at 11:09 p.m. Friday with the subject line "Vice President."

"The Governor has indicated that Obama did not select him as a running mate," it said.

A few hours later, at 3:10 a.m., the Obama campaign sent out a text message to an estimated 3 million supporters nationwide with the official news. But details had already begun to leak as Secret Service agents were dispatched to Biden's home in Delaware, and as Kaine and the others on the shortlist were informed they did not get the job.

In keeping with its secretive approach to the vice presidential rollout, Obama's campaign said it will release no details on how he made his decision. Obama also did not take questions; he has not done so for at least two weeks.